(1) Field of the Invention
The present invention relates to a silicon polyimide precursor composition and a method for preparing a crosslinked and cured material therefrom. More specifically, it relates to a silicon polyimide precursor which is tough and heat-resistant and which has higher hardness than organic compounds and by which undesirable brittleness of inorganic compounds is eliminated, and a method for preparing a crosslinked and cured material therefrom.
(2) Description of the Prior Art
Polyimide resins have maximum heat resistance and low coefficient of thermal expansion considering organic polymer compounds, but these characteristics are inferior to those of inorganic compounds. In order to improve surface hardness, wear resistance and the like of the polyimide resin with the intention of causing these properties to come up to those of inorganic compounds, a means for filling the polyimide resin with an inorganic material is taken. In this case, however, breakage is liable to occur at the interface between the inorganic material and the polyimide resin, and therefore such a means is not always preferable.
On the other hand, silica as an example of the inorganic materials has practically preferable characteristics, e.g., low coefficient of thermal expansion and high hardness in addition to excellent heat resistance, but it is brittle and poor in processability and moldability. For this reason, use applications of silica are limited.
For the purpose of improving the processability and moldability, a variety of compounds have been synthesized in which silicon bonds are partly replaced by alkyl groups.
This technique is successful to some extent, for example, as polydimethylsiloxanes and the like, but such compounds involve drawbacks such as noticeable deterioration of heat resistance, perceptible increase in coefficient of thermal expansion and outstanding degradation of hardness.
For the elimination of these drawbacks, many techniques of chemically combining polyimides with silicon compounds have been reported (e.g., Japanese Patent Laid-open Publication Nos. 143328/1982, 7473/1983 and 13631/1983). These techniques are based on the partial replacement of a diamine component which is a raw material of the polyimide by a polydisiloxane which is terminated with diamines at the both ends.
Japanese Patent Publication No. 32162/1983 has suggested a crosslinked polyimide having a siloxane group which can be prepared by mixing a polyamide acid which is terminated with reactive silicon compounds at both the ends thereof, with a polydisiloxane having hydroxyl groups at both ends thereof, and then heating the mixture.
Furthermore, as a process of forming silica films, there has been suggested a technique of heating a reactive silane such as an alkoxy silane or an acetoxy silane (e.g., Japanese Patent Publication No. 16488/1977 and 20825/1977, Japanese Patent Laid-open Publication Nos. 34258/1980 and 250032/1986, and U.S. Pat. No. 4,408,009).
In recent years, for the purpose of improving mechanical strength and dimensional stability of polyimide, a technique has been suggested which comprises mixing a polymide precursor varnish with a metallic alkoxide or its partial condensate to prepare a uniform solution, and then applying and heating the solution so as to form a polyimide film in which a metallic oxide is dispersed (e.g., Japanese Patent Laid-open Publication Nos. 99234/1988, 99235/1988, 172741/1988, 193935/1988, 199265/1988 and 291924/1988).
Products disclosed in the above-mentioned Japanese Patent Laid-open Publication Nos. 143328/1982, 7473/1983 and 13631/1983 still have disadvantages such as noticeable deterioration of heat resistance, perceptible increase in coefficient of thermal expansion and outstanding degradation of hardness, as in the above-mentioned polydimethylsiloxane and the like.
The compound disclosed in Japanese Patent Publication No. 32162/1983 is excellent in affinity for inorganic compounds but cannot provide any materials having low coefficients of thermal expansion.
With regard to the above-mentioned technique of heating a reactive silane such as alkoxy silane or an acetoxy silane, films synthesized by this technique are very brittle, and the thickness of the films is limited to at most several thousand angstroms.
In the technique disclosed in Japanese Patent Laid-open Publication Nos. 99234/1988, 99235/1988, 172741/1988, 193935/1988, 199265/1988 and the like as mentioned above, the polyimide film is substaitially a mere mixture of the polyimide and the metallic oxide, though it cannot be denied in some cases that the polyimide is partially chemically bonded to the metallic oxide by the addition of a coupling agent.
Therefore, when the content of metallic oxide is increased, the product tends to become opaque and nonuniform, and therefore the amount of metallic oxide is restricted, as a result, characteristics of polyimide can be improved only to some extent. The polyimide product suggested in Japanese Patent Laid-open Publication No. 291924/1988 has a drawback, poor heat resistance.
As discussed above, the conventional techniques have various problems, and thus it has been demanded to develop satisfactory materials having advantages of both inorganic materials and organic materials.